There is known widely a synthetic aperture radar wherein a radio beam is emitted at a constant pulse repetition frequency obliquely directing to a ground from a side-looking radar on board a platform such as an artificial satellite or aircraft, and then time-serial SAR data in the form of an echo signal from the ground is subjected to synthetic processing, whereby range resolution and azimuth resolution comparable to those attained by a large aperture antenna can be achieved effectively using a small aperture antenna. For example, refer to the following papers; JOHN C. KIRK, JR., "A Discussion of Digital Processing in Synthetic Aperture Radar," IEEE TRANSACTIONS OF AEROSPACE AND ELECTRONIC SYSTEMS, VOL. AES-11, NO. 3, MAY 1975, pp. 326-337, and D. J. Bonfield, B. Sc., and J. R. E. Thomas, M. Sc., "Synthetic-aperture-radar real-time processing", IEE PROC., VOL. 127, Pt. F, No. 2, April 1980, pp. 155-162.
As synthetic processing techniques to obtain images from the SAR data, an optical processing technique was typically used conventionally. But, in order to avoid the problem of limitation in mechanical accuracy, stability, etc. of an optical system, there have been recently developed many techniques which can reproduce high-quality images through the digital processing of SAR data.
In SAR, the carrier frequency of the respective pulses transmitted from an SAR antenna undergo linear frequency modulation for the purpose of enlarging a searchable range and increasing range resolution. Therefore, an echo signal (SAR data) for the transmitted pulse signal is made to spread over in the range direction. The echo signal also spreads over in the azimuth direction resulting from the movement of the platform. Time-serial SAR data comprises echoes from the nearest ground to the farthest ground with respect to the platform. The SAR data is sampled at a predetermined rate which is determined by a frequency band width of the transmitted pulse after pulse compression, thereby to produce data in the range direction. This data in the range direction is obtained sequentially for each emission of the transmitted pulse, namely, in the form of data in the azimuth direction. The SAR data thus obtained is stored into a memory in the form of a matrix as range line data for each azimuth direction. The image processing of SAR data is to compress the foregoing received SAR data spreading in both directions and to reproduce the data as well-focused images. Such compression is carried out through two cross-correlating processes. One process is the correlation processing between a conjugate function signal of the transmitted signal and the SAR signal to compress the SAR data in the range direction (range compression). In general, the pulse compression is performed using dispersed delay lines with a frequency modulation characteristic (that is, frequency versus time delay characteristic) opposite to that of the transmitted pulse. The other process is the correlation processing to compress the SAR data in the azimuth direction (azimuth compression). This azimuth compression is performed for each echo signal from each range bin region on the ground through the cross correlating between each echo signal obtained after the range compression and an azimuth reference signal which is determined based on parameters such as the platform's speed, altitude and attitude, and the speed of the target due to rotation of the earth on its axis. Before the azimuth compression processing, there is carried out a range curvature compensation processing to compensate the variation of the range bin including an echo signal from the given target in the azimuth direction (range migration) resulting from a distance variation between the platform and the given target on the ground during the movement of the platform through a very large effective aperture length.
Basically, the foregoing processings must be carried out with high accuracy in order to obtain high quality SAR images. High range resolution is achieved by the range compression through the cross-relating process using a conjugate signal of the transmitted pulse signal. Accordingly, the possibility of obtaining the reproduced images of high quality is dependent on determining the optimum signals for the range curvature compensation and the azimuth reference function.
Heretofore, the determination of those signals has been performed based on the measured parameters such as the speed, altitude and attitude of the moving platform by using sensors on the platform or measuring instruments installed on the ground. Consequently, it is has been difficult to obtain such parameters with high accuracy. Moreover, a large-scale tracking system on the ground has been required for acquisition of the parameters which requires a lot of time.
On the other hand, E. A. Herland has proposed a technique to obtain the optimum azimuth reference function signal directly from the SAR data in a digital manner, in place of the acquisition of the parameters through the complicated processes as mentioned above. Reference is made to a paper by E. A. Herland entitled SOME SAR-PROCESSING RESULTS USING AUTO-FOCUSING", Proceedings of the 3rd SEASAT-SAR Workshop on "SAR Image Quality" held at Frascati, Italy, 11-12, December 1980, (ESASP-172), pp. 19-22. In this proposed technique, the contrasts of the image data picked out in an appropriate range area are measured while varying the speed parameter in a predetermined region, and then both range curvature compensation and azimuth compression are performed based on the speed parameter which makes the contrast maximum. However, since this technique requires the reproduction of images in order to attain the maximum contrast of the picked-out image data for each of the speed parameter values one by one, the number of arithmetic operations is significantly increased with the difficulty reproducing the well-focused, high-quality images rapidly.